The present invention relates to a process for treating residual silicon powder, and more particularly to a process for the production of halosilanes from residual silicon powder.
The present commercial method for manufacturing organohalosilanes is well known and is described in U.S. Pat. No. 2,380,995 issued to Rochow. Rochow discloses the direct reaction of an organo-halide, such as methyl chloride, with silicon particles in order to produce organochlorosilane. Intermixed with such particles of silicon are particles of copper, thereby forming a reactive mass or reactive contact mass. In commercial practice this reaction is generally carried out in one of three types of equipment: the stirred bed type of reactor as described in Sellers, U.S. Pat. No. 2,449,821; the fluidized bed reactor as described in Reed et al., U.S. Pat. No. 2,389,931; or the rotary kiln.
Organotrichlorosilanes and diorganodichlorosilanes are the two basic products of the above described direct process reaction. Such compounds are utilized in the production of organopolysiloxane resins as described in U.S. Pat. Nos. 2,258,218 thru 2,258,222. Other products include organopolysiloxane fluids as described in U.S. Pat. No. 2,469,888 and U.S. Pat. No. 2,469,890 as well as the organopolysiloxane elastomers described in U.S. Pat. No. 2,448,756. Currently, it is preferred to produce the diorganodichlorosilanes commercially because they are generally utilized in producing the linear polysiloxane fluids and polymers used in the production of heat cured rubber elastomers and room temperature vulcanizable silicone rubber compositions of various types.
At a certain stage in the production of the organochlorosilanes by the direct process, the reactive mass or reactive contact mass containing silicon particles becomes less reactive, and it is desirable to replace the reactive contact mass. Thus, the spent or less reactive silicon particles are removed from the reactor, and new silicon particles are inserted therein whereupon the reaction is restarted. When the reactive contact mass becomes less reactive or spent, and it is replaced with a new batch of silicon, the less reactive or spent contact mass is generally referred to as residual silicon, residual silicon powder, residual silicon-containing contact mass or residual contact mass, and these terms are used interchangeably herein. An early solution to the problem was to discard the residual contact mass, however, there is considerable reactive silicon remaining in the residual contact mass, and it is desirable for reasons of economy and for avoiding waste disposal problems, to utilize at least some of the remaining silicon value in the residual contact mass.
Much research has been directed to finding a method for more fully utilizing the residual silicon particles in the reactor used to carry out the direct process synthesis of organochlorosilanes, such that the weight ratio of the organotrichlorosilanes (known as T) to diorganodichlorosilanes (known as D) could be maintained at a desired level for a longer period of time, thereby resulting in the maximum utilization of the silicon particles to produce diorganodichlorosilanes. In the production of organochlorosilanes by the direct process of Rochow, the weight ratio of triorganochlorosilane to diorganochlorosilane (T/D) is desirably about 0.1 during the production of organochlorosilanes and preferably not exceeding about 0.35. However, it has been found that in most commercial manufacturing operations, the ratio will be at about the 0.15 level when the reactor is started with new material, but after a period of reaction, it will rise to an excess of the 0.2 level. One of the breakthroughs in this area is the process disclosed by Dotson in U.S. Pat. No. 3,133,109. Dotson discloses that the silicon particles can be more fully utilized, and the amount of diorganodichlorosilane can be maximized by passing used particles from a fluid bed reactor through an external fluid energy mill. As an alternative to the external fluid energy mill, Dotson also discloses the passing of the used silicon particles that were recycled from the reactor through a plurality of sonic jets located at the base of the reactor to create a comminution of the particles or the breaking up of the silicon particles as a result of the particles striking each other or the walls of the reactor.
It was found that by utilizing the Dotson method there could be obtained from the same amount of silicon particles a larger amount of diorganodichlorosilane such that the ratio could be kept near the desired 0.15 level and would remain less than the 0.35 level for a longer period of time. However, it has also been found that the Dotson process causes underutilization of approximately 12 to 15 percent of the silicon which was introduced into the reactor and which must be removed as waste silicon from the process. It was generally considered that such silicon was spent or exhausted, and therefore no longer capable of being utilized. It is desirable to utilize this waste silicon to prevent the problems normally encountered in the disposal of wastes and for reasons of economy. Furthermore, it is even more desirable to find new processes which can utilize substantially all of the silicon value in residual silicon-containing contact masses after the T/D ratio has risen to an undesirable level.
In U.S. Pat. No. 4,281,149 issued to Shade, there is disclosed a process for treating silicon particles within such a silicon reactor system and thereby improving the usefulness of the silicon metal particles. The Shade process comprises a method of treating silicon particles having generally less than forty microns average diameter size whereupon such particles are abraded to remove the surface coating thereon and whereupon the abraded particles can be returned to the reactor for further utilization.
In U.S. Pat. No. 4,307,242, said patent being incorporated herein by reference, Shah and Ritzer disclose another process for recovering and recycling silicon fines in an organochlorosilane reactor system. The process described by Shah and Ritzer comprises a method for classifying direct process contact mass by particle size whereby the most highly poisoned or impure (spent) silicon particles are separated from the relatively unpoisoned silicon particles, and only such unpoisoned particles are recycled, thereby improving the usefulness of the silicon. Thus, instead of disposing the whole mass of spent silicon fines from the direct process, only a small fraction of the spent silicon fines need be disposed at any given time. In U.S. Pat. No. 4,307,242, fine effluent powder (residual contact mass or residual silicon) is directed to one or more mechanical cyclones for recovery. This fine effluent powder is generally the spent reaction mass from a reactor which produces organotrichlorosilane and diorganodichlorosilane products. Crude T and D products are recovered from the top of the cyclones and these products may contain small amounts of "very fine" entrained particles therein. The remainder of the reaction mass is treated pneumatically in the mechanical cyclones and is directed to a receiving hopper for alternate disposition. It is desirable to obtain useful products at any stage described by Shah and Ritzer, and especially from the relatively unpoisoned fraction of secondary cyclone fines resulting from the classifying process of U.S. Pat. No. 4,307,242. It is also desirable to obtain useful products from silicon and residual silicon-containing contact masses obtained from any other source.
One such useful product which can be obtained from various reactions with silicon, is trichlorosilane also designated herein as monohydrogentrichlorosilane (HSiCl.sub.3). A study on the effect of HCl on the synthesis of methylchlorosilanes is reported on page 138 in Voorhoeve, Organohalosilanes, Precursors to Silicones, published by Elsevier in 1967. Voorhoeve reported reactions carried out with metallurgical grade silicon in the presence of a copper catalyst with various molar ratios of methyl chloride:hydrogen chloride ranging from 6:1 to 1:6. However, these reactions were all carried out at 300.degree. C., and there is no report of any temperature selectivity effect by Voorhoeve.
Barry et al. in U.S. Pat. No. 2,488,487, found increased yields of the "more valuable monoalkyl silicon halides" by the simultaneous introduction of a hydrogen halide, e.g. HCl, along with an alkyl halide upon contact with silicon or an alloy or mixture of silicon with metal at an elevated temperature. Although Barry et al. report a temperature range of 200.degree. C. to 550.degree. C., they found that admixture of hydrogen chloride with the starting methyl chloride resulted in increased yields of monomethyl silicon chlorides, and there is no report of improved or high yields of monohydrogentrihalosilane or of any temperature selectivity effect.
In a method of treating spent metallic reaction masses from the direct process production of organohalosilanes, Nitzche et al. in U.S. Pat. No. 2,803,521, report that the reaction of methyl chloride and silicon at 200.degree. C. to 500.degree. C., usually in the presence of copper or copper chloride, and often with HCl as an added reactant, is the best known and most widely used commercial application of the direct process. According to Nitzche et al., this particular reaction produces various methylchlorosilanes, such as, CH.sub.3 SiCl.sub.3, (CH.sub.3).sub.2 SiCl.sub.2, (CH.sub.3).sub.3 SiCl and CH.sub.3 HSiCl.sub.2. However, Nitzche et al. treat spent (residual) silicon-containing reaction masses by dispersing the reaction mass in water or dilute hydrochloric acid and contact the dispersed mass with a chloride source at a temperature of from 20.degree. C. to 100.degree. C. The silicon particles settle and are separated from the supernatant, and the metal salts in the supernatant solution are precipitated, collected and re-used as fresh catalyst. Nitzche et al. do not suggest preparing monohydrogentrichlorosilane from the spent metallic reaction mass, nor do they report any temperature selectivity effect.
Among the well-known uses for trichlorosilane is a hydrosilation reaction of trichlorosilane (monohydrogentrichlorosilane) to make organofunctional silanes, and the use of trichlorosilane as a feedstock in the manufacture of hyper-pure silicon as discussed in U.K. Patent Application GB 2,028,289 published Mar. 5, 1980 by Woerner et al. Recent developments in the semi-conductor industry have created a growing demand for low-cost, hyper-pure silicon for electronic devices, photovoltaic solar cells and the like. Woerner et al. further indicate that trichlorosilane and silicon tetrachloride are made by the reaction of silicon and hydrogen chloride. Furthermore, Rochow in U.S. Pat. No. 2,380,995 reports the reaction of hydrogen chloride with silicon as described by Combes in Compt. rend. 122, 531 (1896) wherein a mixture of about 80% trichlorosilane and 20% silicon tetrachloride were obtained by passing hydrogen chloride through an iron tube filled with silicon heated to 300.degree. C. to 440.degree. C. Thus, it is desirable to provide additional sources for trichlorosilane, to provide economical methods of making trichlorosilane and to provide a synthesis for making trichlorosilane wherein the yield of trichlorosilane is improved. It is also desirable to provide improved methods of managing residual silicon-containing contact mass obtained from the preparation of organohalosilanes by the direct process reaction.